Structure of Ocean BasinsStructure of Ocean Basins

Chapter 4

Continental shelvesContinental shelves

Part of continentsPart of continents Exposed or not exposed depending on sea levelExposed or not exposed depending on sea level Cut by submarine canyonsCut by submarine canyons Vary in width depending on type of marginVary in width depending on type of margin Accumulate sediment from the continents Accumulate sediment from the continents

(including rivers)(including rivers) Fun fact: 25 % of global continental shelf is in the Fun fact: 25 % of global continental shelf is in the

Bering Sea (very productive fisheries)Bering Sea (very productive fisheries)

Continental SlopeContinental Slope

Separated from the shelf at the shelf break (~ 130 Separated from the shelf at the shelf break (~ 130 m).m).

Continues to an average depth of about 4000 m Continues to an average depth of about 4000 m (range is 3000-8000 m)(range is 3000-8000 m)

Most prominent topographic feature on earthMost prominent topographic feature on earth Marks the end of continents and the beginning of Marks the end of continents and the beginning of

ocean crustocean crust Average width of 20 – 100 kmAverage width of 20 – 100 km Average slope of 4Average slope of 4oo (road grade) - 70 m/km or 370 (road grade) - 70 m/km or 370

feet/milefeet/mile

Continental RiseContinental Rise Sediments from the shelf move down the continental Sediments from the shelf move down the continental

slope and accumulate on the continental rise.slope and accumulate on the continental rise. Most are transported there by turbidity currents or Most are transported there by turbidity currents or

through riverine-derived canyons.through riverine-derived canyons. Width of the rise varies and slope is gradual.Width of the rise varies and slope is gradual. Deep sea currents shape the rises.Deep sea currents shape the rises. Slope is about 1/8 that of the Continental slope (about Slope is about 1/8 that of the Continental slope (about

0.50.5o o or < 10 m/km or < 50 feet/mile)or < 10 m/km or < 50 feet/mile) Rise absent when slope terminates in a trenchRise absent when slope terminates in a trench Conspicuous features of passive (tectonically inactive) Conspicuous features of passive (tectonically inactive)

margins so rare in the Pacificmargins so rare in the Pacific

TrenchesTrenches

Act as sediment traps!Act as sediment traps! Peru-Chile trench – oldest at the S end Peru-Chile trench – oldest at the S end

(shallower because its full of sediments) and (shallower because its full of sediments) and vice versavice versa

Feature of active margin/subduction zoneFeature of active margin/subduction zone

Deep Ocean BasinsDeep Ocean Basins

Mainly, abyssal plains and ocean ridgesMainly, abyssal plains and ocean ridges Also, trenches, seamounts and guyotsAlso, trenches, seamounts and guyots Include more than half the earth’s surfaceInclude more than half the earth’s surface

Abyssal plainAbyssal plain True sea floorTrue sea floor Flat, sediment-covered ocean floorFlat, sediment-covered ocean floor Sediments often more than 1000 m thick (not so featureless Sediments often more than 1000 m thick (not so featureless

underneath sediments)underneath sediments) Begins at the base of the continental riseBegins at the base of the continental rise Very flat except for abyssal hills (which are low, < 1 km Very flat except for abyssal hills (which are low, < 1 km

high, origin unclear), seamounts and guyots and islandshigh, origin unclear), seamounts and guyots and islands 25% of the earth’s surface and 42% of the ocean bottom is 25% of the earth’s surface and 42% of the ocean bottom is

plainplain Most common in the Atlantic and rare in the Pacific where Most common in the Atlantic and rare in the Pacific where

trenches trap sedimentstrenches trap sediments Average of about 4000 – 5500 m depthAverage of about 4000 – 5500 m depth

Abyssal hillsAbyssal hills

Not very high and steepNot very high and steep Protruding ocean floor topography?Protruding ocean floor topography?

Abyssal plain sediments bury old mountains (image generated by Echo sounding)

Important features of the sea floorImportant features of the sea floor TrenchesTrenches

– Extremely deepExtremely deep– Found in association with plate margins (converging); mainly Found in association with plate margins (converging); mainly

in the Pacific because of plates therein the Pacific because of plates there Seamounts and guyotsSeamounts and guyots

– Guyots are flat-topped seamounts (eroded)Guyots are flat-topped seamounts (eroded)– Islands are seamounts that break the sea surfaceIslands are seamounts that break the sea surface

Coral reefsCoral reefs– Most common in Pacific with range 40Most common in Pacific with range 40ooN and 40N and 40ooSS– Require warm water, light (deepest living corals at 100 m), Require warm water, light (deepest living corals at 100 m),

salinity (no freshwater forms) and low sediment load.salinity (no freshwater forms) and low sediment load.– Fragile ecosystemsFragile ecosystems

TrenchesTrenches

Convergence zonesConvergence zones Cold at the bottomCold at the bottom 3 to 6 km deeper than adjacent seafloor3 to 6 km deeper than adjacent seafloor Trenches curved because of geometry of plate Trenches curved because of geometry of plate

interactions on a sphereinteractions on a sphere Trench steeper on the island/continent side than Trench steeper on the island/continent side than

the seaward sidethe seaward side Tectonically activeTectonically active Big sediment trapsBig sediment traps

Island arcsIsland arcs

Parallel to trenchesParallel to trenches Chain of islands behind a trenchChain of islands behind a trench

Seamounts and guyotsSeamounts and guyots

Circular or elipticalCircular or eliptical More than 1 km in reliefMore than 1 km in relief Alone or in groupsAlone or in groups Old hot spots or extinct volcanoes formed at Old hot spots or extinct volcanoes formed at

spreading centersspreading centers Guyots have flat topsGuyots have flat tops

Coral reef formationCoral reef formation

Atolls – around volcanoesAtolls – around volcanoes Fringe reefsFringe reefs Corals grow up to keep in the sunlightCorals grow up to keep in the sunlight Evidence of old reefs at 1400 mEvidence of old reefs at 1400 m

Oceanic RidgesOceanic Ridges

Includes diverging and converging boundaries.Includes diverging and converging boundaries.

15 cm/yr at Iceland

Ocean ridge systemOcean ridge system

Describe plate boundariesDescribe plate boundaries Largest and longest mountain range on earthLargest and longest mountain range on earth Made up of young, basaltic rock at active spreading Made up of young, basaltic rock at active spreading

centerscenters Ridges devoid of sedimentRidges devoid of sediment Oceanic ridges and associated structures account for Oceanic ridges and associated structures account for

22% of the world’s surface (compare to all land which is 22% of the world’s surface (compare to all land which is 29%!)29%!)

< 60% of their length is at the center of basins< 60% of their length is at the center of basins Youngest rocks at spreading centersYoungest rocks at spreading centers Steeper ridges at slower spreading centersSteeper ridges at slower spreading centers

LavasLavas

Lava cooling underwater forms pillow lavasLava cooling underwater forms pillow lavas– Solidifies rapidly because of contact with cold Solidifies rapidly because of contact with cold

seawater; this also slows flowseawater; this also slows flow– Water pressure keeps gases in magma Water pressure keeps gases in magma

“solution” during cooling“solution” during cooling

Lave extruded on landLave extruded on land– Cools more slowly; flows longerCools more slowly; flows longer– Releases gases (sometimes explosively)Releases gases (sometimes explosively)

Transform faultsTransform faults Fractures in the lithosphere along which movement has Fractures in the lithosphere along which movement has

occurredoccurred Characterized by shallow earthquakesCharacterized by shallow earthquakes A result of the earth being a sphereA result of the earth being a sphere Transform faults are the active parts of fracture zonesTransform faults are the active parts of fracture zones Fracture zones extend further away from ridge axis and Fracture zones extend further away from ridge axis and

are evidence of past transform faulting.are evidence of past transform faulting. Lithospheric plate on either side of transform faults Lithospheric plate on either side of transform faults

move in opposite directionsmove in opposite directions Outward sections of fracture zones move in the same Outward sections of fracture zones move in the same

directiondirection

Structure of the mid-Atlantic Ridge between FL and W. AfricaDepressed central rift valley is in blue

Further North and WestSeismic profile of ridge being buried

Hydrothermal ventsHydrothermal vents Discovered only in 1977 along the East Pacific Rise Discovered only in 1977 along the East Pacific Rise

(now found on mid-Atlantic Ridge, in the Sea of Cortez (now found on mid-Atlantic Ridge, in the Sea of Cortez and on the Juan de Fuca Ridge)and on the Juan de Fuca Ridge)

Really hot (~ 350Really hot (~ 350ooC), mineral-rich water (black smokers). C), mineral-rich water (black smokers). Can form chimneys, deposit minerals, affects ocean Can form chimneys, deposit minerals, affects ocean chemistry, etc.chemistry, etc.

Seawater descends through fissures to meet hot rock. Seawater descends through fissures to meet hot rock. Superheated seawater dissolves minerals and escapes Superheated seawater dissolves minerals and escapes upward.upward.

Found in Lake Baikal – future ocean?Found in Lake Baikal – future ocean? Average water temperature is 8 – 16Average water temperature is 8 – 16ooC (bottom ocean C (bottom ocean

water is 4water is 4ooC)C)

Broad continental shelvesTrenchesRidgesGBR

Red SeaGalapagosIceland

Take home pointsTake home points

Continental shelf, break, slope and rise Continental shelf, break, slope and rise (relative slopes)(relative slopes)

Abyssal plainsAbyssal plains Mid-ocean ridge system (describes plate Mid-ocean ridge system (describes plate

boundaries)boundaries) Hydrothermal vents, coral reefs, seamounts Hydrothermal vents, coral reefs, seamounts

and guyotsand guyots

Opening since the Miocene (65 million years ago)